This invention relates generally to flux-gate compasses and, more particularly, to techniques for calibration of flux-gate compasses in moving vehicles. A flux-gate compass is a magnetometer by means of which the direction of the earth's magnetic field may be accurately determined. Flux-gate compasses may be usefully employed on any movable platforms, such as land-based vehicles or ships, and may be integrated into complete navigation systems.
A flux-gate sensor comprises an annular magnetic core and either two or three coils of wire, each of which is wound diametrically about the core. For purposes of explanation, it is easier to assume that there are just two coils, oriented at right-angles to each other. A three-axis magnetometer works on the same principal as the two-axis magnetometer, and there is a mathematical transformation that converts three-axis output signals to the simpler two-axis format. An excitation winding is wrapped about the core at one point in its circumference, and provides a uniform periodic magnetic flux in the core.
When a flux-gate sensor is not subject to any external magnetic field, such as the earth's field, the excitation magnetic flux in the annular core produces zero output voltages at the terminals of both coils. A rigorous mathematical proof of this result can be provided, but it will be apparent that, since the magnetic flux in the annular core passes through each coil twice, in opposite directions, one might expect an induced voltage of zero in each coil.
When the flux-gate sensor is placed in the earth's magnetic field, the magnetic symmetry of the sensor is disturbed and the two coils generate output voltages indicative of the direction of the earth's field. The basic mathematics of the two-axis and three-axis flux-gate sensor are derived in a paper by Timothy J. Peters, entitled "Automobile Navigation Using a Magnetic Flux-Gate Compass," IEEE Trans. on Vehicular Technology, Vol. VT-35, No. 2, May 1986, pp. 41-47. As derived in this paper, the voltage output signals, v.sub.x and v.sub.y, from the two coils, designated the x coil and the y coil, are alternating signals directly proportional to sin .phi. and cos .phi., respectively, where .phi. is the angle between magnetic north and the direction in which the plane of the x coil is aligned. Again, the rigorous mathematical proof of this result is lengthy, but the result agrees with intuition. With the x coil aligned to magnetic north (.phi.=0), one might expect the effect on the x coil to be zero, since the earth's field would not pass axially through the coil, and one might expect the effect on the y coil to be maximized, since the earth's field would be axially aligned with that coil. As the coils are rotated (and .phi. increased), one might also expect the x coil output to increase from zero and the y coil output to decrease from its maximum. These expectations all comport with the variation of the sine function for the x coil output and the cosine function for the y coil output.
In the usual mode of operation of a flux-gate compass, the angle .phi. is computed from: EQU .phi.=tan.sup.-1 (v.sub.x /v.sub.y).
It will be apparent from the sine-cosine relationship of v.sub.x and v.sub.y that, for a perfect flux-gate compass, a plot of V.sub.x versus v.sub.y for all values of .phi. will be circular in shape. Theoretically, this circular characteristic would be obtained if the values of v.sub.x and v.sub.y were measured and plotted while the compass was rotated through 360 degrees. However, various magnetic effects distort the earth's field as seen by the flux-gate compass, and the resultant characteristic is distorted from a perfect circle to an ellipse that is tilted with respect to the x and y axes and is displaced from the origin of the axes. The two most important sources of magnetic distortion are permanent and induced magnetism in the vehicle in which the compass is installed.
U.S. Pat. No. 4,611,293 to Hatch et al. discloses a technique for calibrating a compass to compensate for these distortions. Part of this technique involves transforming the elliptical characteristic to a circle with a radius equal to half the width of the ellipse along its minor axis. The principal drawback of the technique is that the vehicle must be rotated through 360 degrees to perform the calibration. In fact, this is a requirement of all calibration techniques prior to the present invention. Unfortunately, the permanent magnetism effects that distort the compass characteristic are very much dependent on the load carried by the vehicle. If the vehicle loads or unloads a cargo containing magnetic materials, the compass is moved out of calibration. Driving the vehicle in a circle for recalibration may not be convenient, and the compass readings will be suspect until such time as calibration is possible.
It will be appreciated from the foregoing that there is a need for further improvement in compasses employing the flux-gate principle. In particular, what is needed is a flux-gate compass that performs calibration without the need to rotate the vehicle through 360 degrees. The present invention fulfills this need.